Physiology, Homeostasis, and Temperature Regulation

Homeostasis: Maintaining the Internal Environment Tissues, Organs, and Organ Systems Physiological Regulation and Homeostasis Temperature and Life Maintaining Optimal Body Temperature Thermoregulation in Endotherms The Vertebrate Thermostat

Homeostasis: Maintaining the Internal Environment Homeostasis is the maintenance of constant conditions in the internal environment of an organism. Single-celled organisms and simple multicellular animals meet all of their needs by direct exchange of substances with the external environment. Simple, multicellular animal lifestyles are quite limited, however, because no part of their bodies can be more than a few cell layers thick.

EXTERNAL ENVIRONMENT (extracellular fluid) Skin Circulatory system Respiratory system INTERNAL ENVIRONMENT Stomach Foods, salts, and water Digestive system Unabsorbed matter Urinary system Blood (cells + plasma) Organic waste products, salts, and water Cells CO 2 Figure 41.1 Maintaining Internal Stability while on the Go

Tissues, Organs, and Organ Systems Cells grouped together with the same characteristics or specializations are called tissues. The four basic types of tissue are epithelial, connective, muscle, and nervous. An organ is composed of tissues, usually of several different types.

Figure 41.2 Four Types of Tissue

Tissues, Organs, and Organ Systems A discrete structure that carries out a specific function in the body is an organ. Examples include the stomach and the heart. Most organs include all four tissue types. Most organs are part of an organ system, a group of organs that function together.

Physiological Regulation and Homeostasis Homeostasis depends on the ability to regulate the functions of organs and organ systems. Generally, the regulatory systems are the nervous system and the endocrine system. Maintenance of homeostasis is dependent on information received, specifically feedback information that signals any discrepancy between the set point (the particular desired condition or level) and the conditions present. The difference between the set point and the feedback information is the error signal.

Figure 41.4 Control, Regulation, and Feedback

Physiological Regulation and Homeostasis Cells, tissues, and organs are effectors that respond to commands from regulatory systems. Effectors are controlled systems. Regulatory systems obtain, process, and integrate information, then issue commands to controlled systems, which effect change. Regulatory systems receive information as negative feedback, which causes effectors to reduce or reverse a process; or positive feedback which tells a regulatory system to amplify a response. Feedforward information signals the system to change the setpoint.

Temperature and Life Most physiological processes are temperature- sensitive, going faster at higher temperatures. The sensitivity of a physiological process to temperature can be described as a quotient, Q 10. Q 10 is defined as the rate of a reaction at a particular temperature (R T ) divided by the rate of that reaction at a temperature 10°C lower (R T-10 ). Q 10 = R T / R T-10

Figure 41.5 Q 10 and Reaction Rate 3 2 1

Maintaining Optimal Body Temperature Animals may be classified by how they respond to environmental temperatures:  Homeotherms maintain a constant body temperature.  In poikilotherms, body temperature changes when environmental temperature changes.  A third category, heterotherm, fits animals that regulate body temperature at a constant level some of the time, such as hibernating mammals.

Maintaining Optimal Body Temperature Animals may also be classified according to the sources of heat that determine their body temperature:  Ectotherms (most animals aside from mammals and birds) depend on external heat sources to maintain body temperature.  Endotherms (all mammals and birds) regulate body temperature by generating metabolic heat and/or preventing heat loss.

Figure 41.7 Ectotherms nd Endotherms (Part 1)

Figure 41.7 Ectotherms nd Endotherms (Part 2)

Figure “Cold” and “Hot” Fish

Thermoregulation in Endotherms Endotherms respond to environmental temperature change by changing rates of heat production. Within a narrow range of temperatures, the thermoneutral zone, the metabolic rate of endotherms is low and independent of temperature. The metabolic rate of a resting animal within the thermoneutral zone is called the basal metabolic rate (BMR). The BMR of an endotherm is about six times that of an ectotherm of the same size and at the same body temperature.

Thermoregulation in Endotherms Across all the endotherms, BMR per gram of tissue increases as animals get smaller. The reason for this is unknown. It was once thought that larger animals evolved lower metabolic rates to prevent overheating because they have low surface area–volume ratios. However, the relationship between metabolic rate and body mass holds even for very small organisms and for ectotherms, in which overheating is not usually a problem.

Figure The Mouse-to-Elephant Curve (Part 1)

Figure The Mouse-to-Elephant Curve (Part 2)

Thermoregulation in Endotherms Most nonshivering heat production occurs in specialized adipose tissue called brown fat. The tissue looks brown because of its abundant mitochondria and rich blood supply. Brown fat cells have the protein thermogenin which uncouples proton movement from ATP production, so that no ATP is produced, but heat is released. Brown fat is commonly found in newborn infants and animals that hibernate.

Figure Brown Fat

Thermoregulation in Endotherms The coldest environments are almost devoid of ectotherm reptiles or amphibians. Endotherms have many adaptation for reducing heat loss in cold environments:  Reduction of surface-to-volume ratios of the body by short appendages and round body shapes  Thermal insulation by thick layers of fur, feathers, and fat.  Decreasing blood flow to the skin by constricting blood vessels, especially in appendages

Figure Adaptations to Hot and Cold Climates (Part 2)

Thermoregulation in Endotherms In any climate, getting rid of excess heat may also be a problem, especially during exercise. Reduction or loss of fur or hair allows for easier loss of heat from the body to the environment. Seeking contact with water cools the skin because water absorbs heat to a greater capacity than does air. Sweating or panting to increase evaporation provides concomitant cooling (although this benefit may be offset by water loss).

The Vertebrate Thermostat The regulatory system for body temperature in vertebrates can be thought of as a thermostat. This regulator is at the bottom of the brain in a structure called the hypothalamus. The temperature of the hypothalamus itself is the major source of feedback information in many species. Cooling it causes fish and reptiles to seek a warmer environment, and warming it triggers the reverse behavior.

Figure The Hypothalamus Regulates Body Temperature

The Vertebrate Thermostat A fever is a rise in body temperature in response to pyrogens. Exogenous pyrogens come from foreign substances such as invading bacteria or viruses. Endogenous pyrogens are produced by cells of the immune system when they are challenged. Pyrogens cause a rise in the hypothalamic set point, and body temperature rises until it matches the new set point.